1. Field of the Invention
The present invention relates to a corona charger used in an electrophotographic imaging device, such as a laser beam printer.
2. Description of the Related Art
A conventional image forming device, such as a laser beam printer, includes a photosensitive drum and a process unit. The process unit includes a charger, a scanner unit, a developing roller, and a transfer roller, which are disposed in the stated order around the periphery of the photosensitive drum in confronting relation with the peripheral surface of the photosensitive drum. As the photosensitive drum rotates, the drum is uniformly charged by the charger and then exposed to a laser beam emitted from the scanner unit. The laser beam, which is modulated in accordance with image data, scans the charged surface of the photosensitive drum at a high speed to form an electrostatic latent image on the photosensitive drum.
The process unit further includes a toner box holding toner. The toner contained in the toner box is supplied to the developing roller to form a thin film of toner on the surface of the developing roller. Rotation of the developing roller brings the toner into confrontation with the photosensitive drum. The toner is attracted to the static-electric latent image formed on the surface of the photosensitive drum, so that the static-electric latent image is developed into a visible toner image. When the visible toner image moves into confrontation with the transfer roller, the visible toner image is transferred onto a sheet that passes between the photosensitive drum and the transfer roller.
In a xe2x80x9ccleanerlessxe2x80x9d image forming device, toner that remains on the photosensitive drum after transfer of toner onto the sheet is collected by the developing roller. Because the residual toner is collected in this manner, there is no need to provide a blade or other components for cleaning the photosensitive drum or a waste-toner box for holding the toner that was cleaned off from the photosensitive drum. As a result, xe2x80x9ccleanerlessxe2x80x9d image forming devices have a simpler configuration, are more compact, and less expensive.
Sometimes a xe2x80x9ccleanerlessxe2x80x9d image forming device cannot collect all of the residual toner using the developing roller when a great deal of toner remains on the photosensitive drum after visible image transfer. As a result, the residual toner can influence the next visible toner image formed on the photosensitive drum and appear as a ghost image. Therefore, xe2x80x9ccleanerlessxe2x80x9d process units usually also include a cleaning roller in contact with the photosensitive drum at a position downstream with respect to the rotational direction of the photosensitive drum. The cleaning roller is made from a resilient and conductive material. During image forming processes, the cleaning roller is applied with a bias (negative bias) that is lower than the bias at the surface of the photosensitive drum when the visible toner image is transferred onto the sheet, so that the toner remaining on the photosensitive drum is temporarily collected onto the cleaning roller. At periods of non-image transfer, that is, in between successive sheets, the cleaning roller is applied with a bias (positive bias) that is higher than the bias at the surface of the photosensitive drum. Toner that was temporarily collected by the cleaning roller is returned back to the photosensitive drum and the returned toner is then collected by the developing roller.
Japanese Patent-Application Publication No. 9-127844 discloses a conductive brush member for removing paper dust that clings to the photosensitive drum during printing operations. The brush member is disposed in sliding contact with the surface of the cleaning roller and removes paper dust that is mixed in with toner that was temporarily collected by the cleaning roller. The brush member is applied with the same bias as the cleaning roller or applied with a voltage having the same polarity as the toner that was temporarily collected by the cleaning roller.
However, the brush member has a coarse fiber density so that tips of the brush fibers do not uniformly contact the entire outer peripheral surface of the rotating cleaning roller. Therefore, paper dust is not efficiently removed.
The present invention has been made to solve the aforementioned problems, and accordingly it is an object of the invention to provide an image forming device that can efficiently remove paper dust by abutting a secondary roller against the cleaning roller and maintain a high printing quality.
To achieve the above and other objects, there is provided a cleaner that includes a first roller and a second roller. The first roller is for removing foreign matter from a photosensitive body of an image forming device. The second roller is disposed in confrontation with the first roller. An electric field developed between the first roller and the second roller moves foreign matter having a predetermined charge polarity from the surface of the first roller to the surface of the second roller.
With this configuration, after foreign objects are completely removed from the photosensitive member by the first roller, the electric field operating between the first roller and the second roller selects only those foreign objects that have the predetermined charge polarity and moves them to the second roller. This ensures that only desired foreign objects of two types of different-polarity foreign objects are reliably removed.
It is desirable to further provide a slide-contact member that contacts the surface of the second roller and scrapes foreign matter from the surface of the second roller. With this configuration, the foreign objects moved to the second roller can be mechanically removed by the slide-contact member. Therefore, foreign objects will not accumulate on the second roller. A clean surface of the second roller, free of foreign objects, will always confront the first roller so that foreign objects will not be returned back to the first roller. Object removal can be more efficiently performed. It is desirable that the slide-contact member includes a porous material made from at least one of sponge and unwoven fabric. With this configuration, the porous portion of the slide-contact member can mechanically trap and hold foreign objects. The foreign objects can be reliably prevented from returning back to the second roller. Moreover, slide-contact member has a large capacity for holding foreign objects, and so can be used for a long time, which proves to be economical.
It is desirable to further comprising a first voltage application unit for applying voltage to the first roller, a second voltage application unit for applying voltage to the second roller, and a control unit. The control unit controls the first voltage application unit to change polarity of voltage applied to the first roller to one polarity when toner is to be drawn from the photosensitive body and to the opposite polarity when toner is to released onto the photosensitive body. The control unit further controls the second voltage application unit to apply voltage to the second roller that does not change the direction of the electric field between the first roller and the second roller, even if the second voltage application unit changes voltage applied to the first roller.
With this configuration, toner that remained in image regions of the photosensitive member after image transfer is once collected on the first roller and then returned back to the non-image regions for the photosensitive member. As a result, toner is not wasted. Moreover, because the voltage applied to the second roller is controlled so as to not change the electric field between the first roller and the second roller, even if the voltage applied to the second roller is switched. Therefore, when the foreign objects have a different polarity than the toner, the foreign objects that are collected on the first roller with the toner can be selectively removed by the electric field to the second roller. Therefore, the configuration for removing foreign objects is a simple configuration that adds only the second roller to the first roller. Production cost can be reduced.
In this case, it is desirable that the first roller has a higher peripheral speed than the photosensitive body. The first roller and the second roller contact each other at their outer peripheral surfaces at a contact portion and move in the same direction at the contact portion. With this configuration, even if the toner and foreign objects are mixed on the first roller, the electric field can selectively remove only foreign objects without having to mechanically scrape toner off the second roller using a difference in peripheral speed between the first and second rollers. Toner will not be wasted.
When the outer surface of the first roller is a resilient and porous member, then the first roller will have a large surface area that enhances the capacity to remove foreign objects and toner using the electric field. Further, the first roller contacts the photosensitive member with a larger surface area so that the effects of mechanical removal of toner and foreign objects is enhanced.
When the second roller is conductive and has a smooth surface, there is no need to mechanically scrape toner and foreign objects, which have different polarities, from the first roller using the second roller. The foreign objects cling to the second roller by the electric field and the mechanical removal of foreign objects by the slide-contact member can be enhanced.
It is desirable that the first roller removes foreign objects from the photosensitive member using a combination of mechanical scraping-off force and electrostatic drawing force. The second roller selectively draws only paper dust from the first roller using mainly electrostatic drawing force. With this configuration, toner that was collected on the first roller can be returned to the photosensitive member without moving to the second roller and foreign objects that cling to the first roller can be moved to the second roller.
An image forming device that includes a photosensitive body and the inventive cleaner has a more compact size and simpler configuration. Also production costs are reduced.
It is desirable that the image forming device further include a charge unit that develops a uniform charge on the photosensitive member, a developing unit that develops images on the photosensitive member using toner into visible images, and a transfer member that transfers the visible images from the photosensitive member onto sheets. The charge unit, the developing unit, the transfer member, and the first roller of the cleaner device are disposed around the photosensitive drum in this order starting from an upstream side with respect to rotational direction of the photosensitive body. When the image forming device has this configuration, a cleanerless type process unit can be configured. Toner will not be wastefully used. Also, the image forming device is more compact and reasonably priced.